In a press brake for bending a plate form workpiece, it is possible to achieve high precision bending process to obtain a bending angle close to a target bending angle at one bending step by detecting a bending angle of the workpiece on bending and feeding back the detected information to a control unit for controlling a lower limit position of a ram.
Therefore, there have been proposed various methods in detecting bending angle of the workpiece during the bending process.
For example, see Japanese Unexamined Patent Publication (Kokai) No. 57-50217, Japanese Unexamined Patent Publication No. 63-49327, Japanese Unexamined Patent Publication No. 64-2722, Japanese Unexamined Patent Publication No. 64-2723 and so on.
However, in all methods, there are drawbacks in high cost for a die assembly per se due to requirement for special dies, in restriction of the size of the dies or in difficulty of detecting the angle for the workpiece having a cut-out, since the sensor for detecting the bending angle is mounted on the die or the position of the sensor is not fixed.